Design a data structure that supports all following operations in average O(1) time.

  • insert(val): Inserts an item val to the set if not already present.
  • remove(val): Removes an item val from the set if present.
    getRandom: Returns a random element from current set of elements.

Each element must have the same probability of being returned.
Example:

// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);

// Returns false as 2 does not exist in the set.
randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);

// 2 was already in the set, so return false.
randomSet.insert(2);

// Since 1 is the only number in the set, getRandom always return 1.
randomSet.getRandom();

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
import java.util.Random;

public class RandomizedSet {
    private HashMap<Integer, Integer> map;
    private List<Integer> elements;
    private static Random random = new Random();

    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = new HashMap<>();
        elements = new ArrayList<>();
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if (!map.containsKey(val)) {
            map.put(val, elements.size());
            elements.add(val);
            return true;
        } 
        return false;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (map.containsKey(val)) {
            int loc = map.get(val);
            map.remove(val, loc);
            
            if (loc != elements.size() - 1) {
                int end = elements.get(elements.size() - 1);
                map.put(end, loc);
                elements.set(loc, end);
            }
            elements.remove(elements.size() - 1);
            return true;
        }
        return false;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return elements.get(random.nextInt(elements.size()));
    }
}

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */

Another solution using two hash map

link